1. Field of the Invention
This invention relates to a log carriage for supporting logs during scanning and sawing thereof.
2. Description of the Prior Art
Log carriages are used to secure logs in a desired orientation and move them generally parallel to their longitudinal axes during the sawing operation. The carriages typically have wheels or rollers which ride on guide rails extending generally parallel to the log. The logs are held on the carriage by pointed projections known as "dogs". In the past, the dogs have usually engaged the sides of the log.
After a section of the log has been sawn, it is usually desirable to rotate it ninety degrees or one hundred and eighty degrees from the sawn face and then continue to saw. The dogs are first removed from the log. The log is then rotated the desired amount and the dogs engage the log again to hold it in a second fixed position.
Using log carriages of this type has several disadvantages. First, the log cannot be spin-scanned to determine its profile and cannot be x-ray scanned for internal defects. Second, the sharp dogs damage the outside surface of the log where the best lumber is found. This is especially important for large diameter logs where the outside is the most valuable part. Also, in some cases such carriages induce bending of the log which results in distorted boards when they are sawn from the log.
Because of these disadvantages, end-dogging log carriages were developed where the log is held by opposing dogs which engage the ends of the log only. However, there have been certain disadvantages or limitations with respect to the type of end-dogging carriages developed in the past. For example, in some prior art devices the log is held in position for sawing by an hydraulic brake. After sawing is completed at this position, the log is rotated ninety degrees and the brake is engaged again. However, this rotation cannot be accomplished with extreme accuracy in some prior art end-dogging log carriages because of mechanical backlash in the components of the carriage for example. Such systems are often not accurate even if such accuracy is claimed. For example, lock valves used on hydraulic systems often leak. Even the loss of two or three drops of hydraulic fluid can change the postion of a cylinder. Also reed switchs are not accurate because of mechanical backlash in chains or sprockets for example.
Another problem is that some prior art end-doggers do not support the logs fully by means of the end-dogs. For example, rolls or cylinders may be used to support the log between the end-dogs. Supporting the logs in this manner makes it more difficult to manipulate the log and hold it at a desired orientation for the sawing operation.
Scanning of logs prior to sawing is a common procedure to maximize yield. The scan of the log determines the best way of cutting the log to get the greatest yield, particularly of the more valuable cuts of timber. In many types of log carriages, including end-dogging log carriages, the log is released after the scanning operation is performed. It is reengaged by the dogs prior to the sawing operation. The log may also be released from the dogs between successive sawing positions of the log. This means that the point of reference is lost each time the log is released. Thus such log carriages are not well adapted to automated scanning and sawing of the log, because the logs do not maintain any fixed orientation with respect to any component of the carriage which could provide a fixed reference for scanning and handling.